DESCRIPTION: The goal of the work in this proposal is a characterization of the Msx1 gene in the mouse. Msx1 encodes a homeodomain protein which when deleted causes cranial facial defects in mice. During the past funding period, Abate identified a DNA binding consensus for Msx1 and produced convincing data that the gene functions as a repressor of transcription in vitro. Moreover, she showed that this repressor activity does not require Msx=s binding to its target DNA, although it does require an intact Msx homeodomain. Abate argues that Msx's transcriptional specificity reflects an interaction with other proteins in a large Msx complex and has identified several possible candidates. The future analysis of these interacting proteins and the resultant specificity of the complex provide the basis for the proposed experiments in this application. To evaluate the in vivo significance of newly identified potential candidates she has established cell lines in which Msx can be induced under TET control and its effects assayed by activation of MyoD. As a first step in developing a biologically relevant assay for Msx function, she has used the Tabin retroviral expression systems to overexpress Msx in the Chick limb. Overexpression at stage 10 causes wide spread defects in limb development. More useful for the present proposal is her observation that injection at stage 17 blocks differentiation of feather follicles. Abate proposes to develop this overexpression phenotype as a quantitative assay for function.